Rome, Italy (CNN) -- Race organizers Friday said a balloon carrying two missing Americans over the Adriatic Sea off Italy made a rapid descent during rough weather, conditions which would make their survival "unlikely."

Race officials from the the international gas-balloon competition said they lost contact with the balloon of Richard Abruzzo of New Mexico and Dr. Carol Rymer-Davis of Colorado as it was over the sea shortly after 8 a.m. local time (2 a.m. ET) Wednesday.

Officials looked at transponder readings from air traffic control and made calculations from the height of the balloon, the time and its location, said flight director Don Cameron. The data showed the balloon had a "moderate descent rate initially which then increased into a high rate of descent, to around 50 miles per hour."

"It is the opinion of the Gordon Bennett 2010 Flight Control Team that the balloon appears to have suffered a sudden and unexpected failure," the statement read. "The cause of this tragedy is still being examined."

It asked that the families not be contacted.

The search for the pair continued Friday as Italian authorities took photos of debris found in the sea and forwarded them.

"There have been some pieces of debris found in the sea," said Cameron. "We received pictures of the debris by e-mail from the search and rescue team, and after examination decided none of what we've seen so far was from the balloon."

Italian Coast Guard Warrant Office Massimo Maccheroni said a search boat found the debris. An underwater vessel was en route and should reach the scene soon and a military helicopter was in the area, he said.

The pieces of debris are small and one resembles a cord, Maccheroni said. The items were found near the town of Vieste.

Two U.S. aircraft assisted eight Italian vessels and aircraft in the search, which covers 3,600 square miles. The search will continue Saturday.

Abruzzo's wife, Nancy, said earlier she was hopeful.

"We've had other challenges before, but we've never lost contact for this long. But they are smart strong and capable. And they are survivors," she said.

Severe thunderstorms were in the vicinity of the balloon -- which launched from the United Kingdom on Saturday -- when race officials received the last signal from the balloon's tracker device, Cameron said earlier in the week.

Maccheroni listed three hypotheses on what happened: Lightning struck the balloon and it exploded; the balloon suffered a failure and went down quickly into the ocean; or it dropped slowly, making survivability more likely.

The balloon was equipped with a satellite phone, VHF radios, a radar transponder and two mobile phones, but efforts to contact Abruzzo and Rymer-Davis so far have been unsuccessful.

The balloon also had survival suits, life jackets and two single-person life rafts, according to Cameron.

"We have not found any safety device [lifeboat], and one can last only ... so long at sea without one," Maccheroni said.

The 20-balloon race began Saturday night near Bristol, England, with pilots vying to fly the farthest from the start point. The balloons flew over the English Channel and continued south across France, with most of the balloons ending up landing in France or northern Spain.

But six, including the one piloted by Abruzzo and Rymer-Davis, ventured over the Mediterranean. Five of them landed in various European countries Tuesday and Wednesday.

Rymer-Davis is a radiologist in Denver, Colorado, according to KUSA. She and Abruzzo won the 2004 Gordon Bennett race, which took off that year from France.